# How to Insert Into A List With Slicing In Python

## Used to solve Leet Codeās Duplicate Zeroes Puzzle

2 min readSep 14, 2021

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The aim of this pageš is to utilize Python slicing to insert a data structure into a list at a given index. Slicing indexes operate as a half-open range ā the first item is included, the last is not:

`>>> l = [1,2,3,4]`

>>> l[0:1]

[1]

- to insert at a given point, start pointing to the item you want the new structure to start with. then, use the same index value after
- e.g.
`[0:0]`

to insert a sublist of a random length in the very beginning of a list - the identical values in ([0:0]) are not specifying the span, they only point to the point of entry
- in a stupid example below I want to add
`666`

at the beginning of the list`[1,2,3,4]`

`>>> l = [1,2,3,4]`

>>> l[0:0] = [6,6,6,]

>>> l

[6, 6, 6, 1, 2, 3, 4]

>>>

## EXAMPLE: LEET CODEāS DUPLICATE ZEROS PUZZLE

- Iāve used the technique to easily solve https://leetcode.com/problems/duplicate-zeros/ which seems rather challenging in an official solution
- It goes like this: Given a fixed-length integer array
`arr`

, duplicate each occurrence of zero, shifting the remaining elements to the right. - Note that elements beyond the length of the original array are
**not written**. - Do the above modifications to the input array
**in place**and do not return anything.

`# EXAMPLE`

Input: arr = [1,0,2,3,0,4,5,0]

Output: [1,0,0,2,3,0,0,4]

Explanation: After calling your function, the input array is modified to: [1,0,0,2,3,0,0,4]

- I only insert at the point where the zero is found with inserting with slices
- Then, I remove the last item by with
`pop()`

- Finally, I move the
`i`

pointer by two to reflect the insertion as we're mutating a collection that's also being iterated upon

`# SOLUTION`

def duplicateZeros(self, arr: List[int]) -> None:

i = 0

while i < len(arr):

if arr[i] == 0:

arr[i:i] = [0]

arr.pop()

i += 2

else:

i += 1